1. Field of the Invention
The present invention relates to polyimide powder which gives polyimide powder molded bodies that maintain a high level of heat resistance with particularly high flexural strength and tensile strength and high elongation, as well as to polyimide powder molded bodies and to a process for their production.
2. Description of the Related Art
Pyromellitic acid-based polyimide powder molded bodies obtained from a pyromellitic acid component and 4,4xe2x80x2-diaminodiphenyl ether have been widely used in the prior art as polyimide powder molded bodies because of their high toughness and satisfactory cutting workability.
However, pyromellitic acid-based polyimide molded bodies have high moisture absorption, considerable out gas and low chemical resistance and dimensional stability.
3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid-based polyimide powder molded bodies have therefore been proposed.
Examples of such 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid-based polyimide powder molded bodies are described, for example, in Japanese Unexamined Patent Publication No. 57-200453, wherein there are obtained heated/compressed molded bodies of relatively large-sized aromatic polyimide powder with an imidation rate of 95% or greater obtained by polymerization and imidation of a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid component and an aromatic diamine component in N-methyl-2-pyrrolidone.
Also, Japanese Examined Patent Publication No. 39-22196 describes polyimide powder obtained by synthesizing a polyimide precursor in an amide-based solvent and reprecipitating the solution with a mixture of toluene, pyridine and acetic anhydride for imidation, and it is indicated that the polyimide powder obtained by this process gives molded bodies with satisfactory cohesion and high density. In addition, Japanese Unexamined Patent Publication No. 7-33873 describes a process for synthesis of a polyimide precursor powder in a water-soluble ketone, whereby removal of the solvent is facilitated, production of polyimide precursor powder with a high degree of polymerization is possible, and the molded bodies obtained from the polyimide precursor exhibit excellent dynamic properties and low residual solvent.
However, since it is difficult by the process described in Japanese Examined Patent Publication No. 39-22196 mentioned above to produce fine polyimide powder, and partial lump formation hampers efforts to achieve a uniform particle size, the molded bodies formed from the powder tend to be irregular. Moreover, employing the process of Japanese Unexamined Patent Publication No. 7-33873 tends to give fine powder with poor filtering properties, while the powder easily coheres together after filtration and during imidation in the solid phase. Also, polyimide powder obtained from 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride and para-phenylenediamine as the aromatic diamine component using this process has insufficient molecular weight, resulting in brittle molded bodies.
It is therefore an object of the present invention to provide polyimide powder molded bodies with excellent heat resistance and satisfactory mechanical properties which are obtained from polyimide powder whose particle size can be controlled and which exhibits satisfactory filtering properties and no cohesion between particles, as well as polyimide powder as the starting material therefor and a process for their production.
In other words, the present invention provides a process for production of polyimide powder which comprises reacting a biphenyltetracarboxylic dianhydride and an aromatic diamine in an amide-based solvent optionally containing a water-soluble ketone, in the presence of an imidazole at 1-100 equivalent percent and preferably 6-100 equivalent percent based on the carboxylic acid content of the polyimide precursor, separating and collecting the produced polyimide precursor powder from a water-soluble ketone solvent containing 3-30 wt % of an amide-based solvent, and heating the separated and collected polyimide precursor powder to an imidation rate of 90% or greater.
The invention further provides polyimide powder obtained by the aforementioned process.
The invention still further provides biphenyltetracarboxylic acid-based polyimide powder molded bodies having a density of at least 1.3 g/mm3, a tensile strength of at least 800 Kg/cm2 and a tensile break elongation of at least 10%, obtained by subjecting the aforementioned polyimide powder to heat and pressure in a die either simultaneously or separately.
The invention still further provides a process for production of polyimide powder molded bodies whereby the aforementioned polyimide powder is packed into a die and subjected to heat in a range of about 300-550xc2x0 C. and pressure in a range of about 100-5000 Kg/cm2 either simultaneously or separately for molding.
Preferred embodiments of the invention are listed below.
1) The aforementioned process for production of polyimide powder wherein the 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride unit is included at 30 mole percent or greater.
2) The aforementioned process for production of polyimide powder molded bodies wherein the molding step is carried out by compression molding, wet CIP or dry CIP (CIP: Cold Isostatic Pressure) or HIP (HIP: Hot Isostatic Pressure).
According to the invention, the tetracarboxylic acid component of the polyimide may be 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride and/or 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride, and is preferably a biphenyltetracarboxylic dianhydride containing at least 30 mole percent of 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride.
Part of the biphenyltetracarboxylic dianhydride may be replaced with another aromatic tetracarboxylic dianhydride such as pyromellitic dianhydride, 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic dianhydride, 2,2xe2x80x2-bis(3,4-dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride or bis(3,4-dicarboxyphenyl)ether dianhydride, so long as the effect of the invention is not hindered.
The diamine component used may be any aromatic diamine such as, for example, para-phenylenediamine (p-phenylenediamine), 4,4xe2x80x2-diaminodiphenyl ether, 1,3-bis(4-aminophenoxy)benzene, 4,4xe2x80x2-diaminodiphenylpropane, 4,4xe2x80x2-diaminodiphenylethane, 4,4xe2x80x2-diaminodiphenylmethane, bis[4-(4-aminophenoxy)phenyl]propane, 2,2xe2x80x2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane or bis[4-(4-aminophenoxy)phenyl]ether, among which para-phenylenediamine (p-phenylenediamine) and 4,4xe2x80x2-diaminodiphenyl ether are preferably used.
According to the invention, it is necessary to react a biphenyltetracarboxylic dianhydride with an aromatic diamine in an amide-based solvent optionally containing a water-soluble ketone, in the presence of an imidazole at 1-100 equivalent percent and preferably 6-100 equivalent percent based on the carboxylic acid content of the polyimide precursor, separate and collect the produced polyimide precursor from a water-soluble ketone solvent containing 3-30 wt % of an amide-based solvent, and heat the separated and collected polyimide precursor powder to an imidation rate of 90% or greater to obtain the polyimide powder.
According to the invention, a biphenyltetracarboxylic dianhydride may be reacted with a molar equivalent of an aromatic diamine for about 30 minutes to 24 hours at 10-40xc2x0 C. in a water-soluble ketone solvent containing 3-30 wt % of an amide-based solvent in the presence of the necessary amount of an imidazole, and then the high-molecularized precipitated polyimide precursor separated and collected. In this case, the total of the biphenyltetracarboxylic dianhydride and aromatic diamine in the solution is preferably 1-20 wt %.
Alternatively, a biphenyltetracarboxylic dianhydride may be reacted with a molar equivalent of an aromatic diamine for about 30 minutes to 24 hours at 10-40xc2x0 C. in an amide-based solvent in the presence of the necessary amount of an imidazole to produce a high molecular polyimide precursor, and then a water-soluble ketone added to the reaction mixture in an amount giving an amide-based solvent proportion of 3-30 wt % of the solvent component to precipitate powder, and the polyimide precursor powder separated and collected. In this case, the total of the biphenyltetracarboxylic dianhydride and aromatic diamine in the solution is preferably 1-20 wt %, and water is preferably included in the amide-based solvent at 1-20 wt %.
As amide-based solvents there may be mentioned N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and N-methylcaprolactam, among which N-methyl-2-pyrrolidone and N,N-dimethylacetamide are preferably used.
As water-soluble ketones there may be mentioned acetone, xcex3-butyrolactone, methylethyl ketone, methylisobutyl ketone and cyclohexanone.
According to the invention, an imidazole must be present during the reaction between the biphenyltetracarboxylic dianhydride and the aromatic diamine.
The amount of the imidazole is preferably 1-100 equivalent percent and especially 6-100 equivalent percent based on the carboxylic acid content of the polyimide precursor.
If the amount of the imidazole is below this range the polyimide powder will not have sufficient molecular weight and the molded body will therefore be brittle, while if the amount of the imidazole is above this range, no further effect is exhibited, resulting in an economic disadvantage.
The imidazole may be added at the initial stage when the other components are added, during the reaction, or in a final stage of the reaction, but it is preferably added in a final stage of the reaction.
As imidazoles there may be mentioned 1,2-dimethylimidazole, imidazole, benzimidazole, N-methylimidazole, N-benzyl-2-methylimidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole and 5-methylbenzimidazole. A portion of the imidazole may be replaced with isoquinoline, 3,5-dimethylpyridine, 3,4-dimethylpyridine, 2,5-dimethylpyridine, 2,4-dimethylpyridine, 4-n-propylpyridine or the like.
According to the invention, the polyimide precursor powder precipitated by the method described above may be filtered and washed with a water-soluble ketone to separate and collect the polyimide precursor powder, and this then heated to obtain polyimide powder with an imidation rate of 90% or greater.
The heating may be carried out at no higher than 250xc2x0 C. under either normal pressure or reduced pressure, and preferably at no higher than 200xc2x0 C., to produce a dry state with a weight reduction of preferably no greater than 1% and especially no greater than 0.5% with heating for one hour at 350xc2x0 C.
According to the invention, the aforementioned polyimide powder is packed into a die and subjected to heat in a range of about 300-550xc2x0 C. and pressure in a range of about 100-5000 Kg/cm2 either simultaneously or separately to form a biphenyltetracarboxylic acid-based polyimide powder molded body with a density of 1.3 g/mm3 or greater, a tensile strength of 800 Kg/cm2 or greater and a tensile break elongation of 10% or greater.
The molding step may be accomplished by compression molding, wet CIP or dry CIP (CIP: Cold Isostatic Pressure) or HIP (HIP: Hot Isostatic Pressure).
For production of the aforementioned powder molded body, a filler of any type, for example, an inorganic filler such as artificial diamond, silica, mica, kaolin, boron nitride, aluminum oxide, iron oxide, graphite, molybdenum sulfide or iron sulfide, or an organic filler such as a fluorine resin, may be mixed with the polyimide powder.
The filler addition may be accomplished by mixing using any internal addition or external addition method.
Polyimide molded bodies obtained by the process of the invention exhibit good uniformity, satisfactory elongation and high productivity without loss of the excellent heat resistance and dimensional stability of conventional publicly known biphenyltetracarboxylic acid-based polyimide powder molded bodies.
Examples of the invention will now be described.
In the examples, the imidation rates are expressed as the proportion (%) of the IR spectrum absorbance ratio [absorbance (1780 cmxe2x88x921)/absorbance (1720 cmxe2x88x921)] for the test polyimide with respect to the absorbance ratio for a polyimide with 100% ring closure.
Also, the glass transition temperature (Tg) of each polyimide is the value measured with an SSC5200 RDSC220C by Seiko Instruments Co., Ltd. at a temperature elevating rate of 10xc2x0 C.
The abbreviations used in the descriptions which follow refer to the compounds listed below.
a-BPDA: 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride
s-BPDA: 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride
PPD: p-phenylenediamine
ODA: 4,4xe2x80x2-diaminodiphenyl ether
DMZ: 1,2-dimethylimidazole
2MZ: 2-methylimidazole
NMP: N-methyl-2-pyrrolidone
DMAc: N,N-dimethylacetamide